The present application claims priority under 35 U.S.C xc2xa7119 based upon Swiss Patent Application No. 2002 0188/02 filed on Feb. 1, 2002 which is incorporated herein by reference.
The invention concerns a method for the calibration of a Wire Bonder.
A Wire Bonder is a machine with which wire connections are made to semiconductor chips after they have been mounted on a substrate. The Wire Bonder has a capillary which is clamped to the tip of a horn. The capillary serves to secure the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On making the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding from the capillary is first melted into a ball. Afterwards, the wire ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonics. In doing so, ultrasonics are applied to the horn from an ultrasonic transducer. This process is known as ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the substrate. This last process is known as wedge bonding. After securing the wire to the connection point on the substrate, the wire is torn off and the next bond cycle can begin.
The ball bonding is influenced by various factors. In order to achieve bond connections of a predetermined quality, the adequate values of several physical and/or technical parameters must be determined for a particular process. Examples of such parameters are the bond force, that is the force which the capillary exerts on the ball or the connection point of the semiconductor chip during the bonding process, or the amplitude of the alternating current which is applied to the ultrasonic transducer of the horn.
The distances between the connection points on the semiconductor chips, known in the art as xe2x80x9cpitchxe2x80x9d, are becoming increasingly smaller. Today, in the Fine Pitch field, one already tends towards a pitch of only 50 xcexcm. This means that the dimensions of the capillary in the area of its tip are also becoming increasingly smaller in order that the capillary does not come into contact with the already bonded wires. With the increasingly smaller dimensions of the tip of the capillary, the influence of unavoidable manufacturing tolerances on the mechanical characteristics of the capillary become greater. With bonding, the capillary wears out so that from time to time it has to be replaced by a new capillary. Today, in order to achieve reliable bonding results even in the Fine Pitch field without having to recalibrate the Wire Bonder with time consuming work after every capillary change, the capillaries are selected according to strict geometrical criteria.
The object of the invention is to develop a method for the calibration of a Wire Bonder which guarantees that, in mass production, semiconductor chips are wired under the same process conditions before and after a capillary change.
A further task which is set in mass production is the transfer of the optimum parameters found on one Wire Bonder to another Wire Bonder. The invention should also offer a solution for this task and support the recipe transfer from Wire Bonder to Wire Bonder in a simple and robust manner.
Each Wire Bonder has a capillary clamped to a horn. Ultrasonics is applied to the horn by an ultrasonic transducer, whereby the ultrasonic transducer is controlled by means of a parameter P. The parameter P is preferably the current which flows through the ultrasonic transducer. The parameter P can however also be the amplitude of the alternating voltage applied to the ultrasonic transducer or the power or another quantity which controls the ultrasonic transducer.
As a rule, on capillary change, the oscillating behaviour of the capillary tip changes because every capillary has somewhat different characteristics and is also slightly differently clamped onto the horn. The endeavoured aim of the named tasks consists in measuring the basic influential quantities of the capillary or the oscillation system formed by the horn and the capillary which have a fundamental influence on the bonding process and using the knowledge gained to reset the relevant bond parameters of the Wire Bonder after a capillary change according to the mechanical characteristics of the new capillary and only starting production with the new capillary after this.
The invention is based on the knowledge that the mechanical characteristics of the tip of the capillary have a strong influence on the ultrasonic force which the capillary exerts on the ball bond. Because the dimensions of the capillary in the area of its tip are becoming smaller and smaller, unavoidable manufacturing tolerances also cause increasing variations in the rigidity from capillary to capillary. The invention provides a solution as to how these variations in rigidity can be compensated.
During bonding, a predefined bond force is applied to the capillary. The tip of the capillary therefore presses in vertical direction against the ball bond which is clamped between the capillary and the connection point of the substrate. When ultrasonics is applied to the horn, then stationary ultrasonic waves are formed in the horn and in the capillary. Because the capillary is pressed against the ball bond, its tip can not oscillate freely. The tip of the capillary therefore exerts a force directed in horizontal direction, a so-called tangential force FT, on the ball bond. This tangential force FT is a function of the deflection AH(t) of the tip of the horn in relation to the tip of the capillary, whereby the parameter t designates the time. The tangential force FT(t) is an alternating force FT(t)=FT0*cos((xcfx89t), which oscillates with the frequency xcfx89 of the ultrasonic waves.
The amplitude AH of the oscillation of the horn at the clamping point of the capillary relative to the tip of the capillary lies typically in the range of 0.1-4 xcexcm and is therefore small in relation to the length of the capillary of typically 11 millimetres. The amplitude AH is also small in relation to the length of the thinnest part of the capillary, namely the tapering at the tip of the capillary. The capillary therefore behaves almost like a spring, ie, the amplitude FT0 of the tangential force FT(t) is in good approximation proportional to the amplitude AH of the oscillations of the horn at the clamping point of the capillary relative to the tip of the capillary:
FT0=k*AH,xe2x80x83xe2x80x83(1)
whereby the quantity k designates a constant which is dependent on the mechanical characteristics of the capillary: The constant k is a measure of the flexural strength of the capillary. The amplitude FT0 of the tangential force is therefore fundamentally dependent on two quantities, namely the amplitude AH, which is controlled by the ultrasonic transducer, and on the flexural strength of the capillary.
The solution of the named tasks now exists in determining the flexural strength for each capillary and, after each capillary change, adapting the parameter P, which controls the ultrasonic transducer, to the determined flexural strength of the capillary so that the tangential force exerted on the ball bond by the respective capillary is equally great before and after a capillary change.
When setting up for bonding a new product, the optimum values for various parameters such as bond force, parameter P for control of the ultrasonic transducer, etc, must first be determined. In the following, it is explained how the parameter P for control of the ultrasonic transducer is reset after a capillary change. The parameter P is, for example, the amplitude I0 of the alternating current I which is applied to the ultrasonic transducer. A linear relationship exists between the amplitude AH and the amplitude I0 of the alternating current: AH=xcex1*I0, whereby the quantity xcex1 is a Wire Bonder dependent constant which can be determined, for example, by means of a calibration in accordance with the method given in the European patent EP 498 936. Under the prerequisite that the clamping of the capillary in the horn has no or only little influence on the quantity AH, the amplitude FT0 of the tangential force therefore results in
FT0=k*AH=k*xcex1*I0.xe2x80x83xe2x80x83(2)
Apart from influences of the clamping of the capillary in the horn, the amplitude of the tangential force FT produced by the Wire Bonder is therefore the same before and after a change from a first capillary, the flexural strength of which is characterised by the value k1, to a second capillary, the flexural strength of which is characterised by the value k2 when the Wire Bonder is operated after the capillary change with the value
P2=I0,2=k1/k2*I0,1=k1/k2*P1xe2x80x83xe2x80x83(3)
Because, with this correction, only the relationship k1/k2 is dealt with, it suffices when the flexural strength is not absolute but is only known as far as a proportional constant.
From the equation (1) it can be seen that, apart from the flexural strength of the respective capillary, the amplitude AH of the oscillations of the horn at the clamping point of the capillary relative to the tip of the capillary also influences the tangential force exerted on the ball bond. In order to also correct the influences of the clamping of the respective capillary on the horn as well as the distance L from the tip of the capillary to the clamping point on the horn which varies from clamp to clamp, the amplitude of the alternating current flowing through the ultrasonic transducer is preferably also corrected according to the amplitude AH. In this case therefore, on the one hand, for the first capillary the flexural strength k1 is determined and, on the other hand, the amplitude AH1 of the oscillations of the horn is measured relative to the tip of the first capillary. Analogously, for the second capillary, the flexural strength k2 and the corresponding amplitude AH2 are determined. After the capillary change, parameter P2 which is given by:
P2=I0,2=k1/k2*AH1/AH2*I0,1=k1/k2*AH1/AH2*P1.xe2x80x83xe2x80x83(4)
is then applied to the ultrasonic transducer. Measurement of the amplitude AH during the bonding process is relatively awkward because, during bonding, the ball bond starts relatively soon to slide back and forth on the connecting point. However, it has been shown that, instead of the amplitude AH, the amplitude AC of the oscillations of the capillary in the area underneath the clamping point on the horn or the amplitude AS of the tip of the capillary can be used while the capillary oscillates freely in the air. For parameter P2 one then gets:
P2=k1/k2*AC1/AC2*P1 orxe2x80x83xe2x80x83(5)
P2=k1/k2*AS1/AS2*P1,xe2x80x83xe2x80x83(6)
whereby the quantities AC1 and AS1 designate the corresponding amplitudes before the capillary change and the quantities AC2 and AS2 designate the corresponding amplitudes after the capillary change. Of course it is important that the measurement of these amplitudes takes place with the capillary in the same position. Methods for measuring the amplitude of the freely oscillating capillary are known from the European patent EP 498 936 and from the Japanese patent application JP 10-209199. However, these documents assume that the oscillations of the capillary run parallel to the longitudinal direction of the horn. This is however not always the case: The amplitude of the oscillations of the capillary in a horizontal direction orthogonal to the longitudinal direction can amount to 30% of the amplitude of the oscillations of the capillary parallel to the longitudinal direction of the horn. With the measurement it must therefore be observed that the actual amplitude of the oscillations of the capillary is measured and not just a component of it.
When one of the amplitudes AH1, AC1 or AS1 is measured as the amplitude A(W1), then the invention can also be used for recipe transfer from a first Wire Bonder W1 to a second Wire Bonder W2. The flexural strengths k1 and k2 of the used capillaries are determined as explained above. Furthermore, on the first Wire Bonder W1 the amplitude A(W1) of the capillary is determined when a predefined value P0 of the parameter P is applied to the ultrasonic transducer. The value P(W1) designates the value of the parameter P which is applied to the ultrasonic transducer of the first Wire Bonder W1 with the setup bonding process. On the second Wire Bonder W2 the corresponding amplitude AH2, AC2 or AS2 is determined as amplitude A(W2) of the capillary when the value P0 of the parameter P is applied to the ultrasonic transducer of the second Wire Bonder. The second Wire Bonder W2 is then operated with the value P(W2) of the parameters P which is given by P(W2)=k1/k2*A(W1)/A(W2)*P(W1). The value P(W1) designates the value of the parameter P which is applied to the ultrasonic transducer of the first Wire Bonder W1 with the setup bonding process.
It has now been shown that a further geometrical quantity exerts an influence on the tangential force, namely the diameter of the longitudinal drill hole of the capillary which guides the wire in the area of the tip. The longitudinal drill hole of the capillary has a constant diameter H in the lower area which widens out at the outlet for reasons which are irrelevant for the present invention. Because, on bonding, only that part of the ball bond located outside the longitudinal drill hole is deformed and not the part located inside, the portion of the deformed part of the ball bond reduces with increasing diameter H of the longitudinal drill hole. Therefore, after a capillary change, it is advantageous to apply the parameter P2 which is given by:
P2=k1/k2*A1/A2*H12/H22*P1,xe2x80x83xe2x80x83(7)
to the ultrasonic transducer whereby k1 designates the flexural strength, A1 one of the above-mentioned amplitudes AH1, AC1 or AS1 of the freely oscillating capillary and H1 the diameter of the longitudinal drill hole of the capillary before the capillary change and k2, A2 and H2 the corresponding values of the capillary after the capillary change.
The correction factors explained above have proven themselves in practice. However, there are also cases where the correction factors have a more general dependency so that the parameter P2 which, according to the model, is given by:
P2=g(k1, k2)*P1xe2x80x83xe2x80x83(8),
or
P2=g(k1, k2, A1, A2)*P1xe2x80x83xe2x80x83(9),
or
P2=g(k1, k2, A1, A2, H1, H2)*P1xe2x80x83xe2x80x83(10)
is applied to the ultrasonic transducer. In the following, different methods are explained based on the figures with which the flexural strength or an estimated value for the flexural strength is determined by means of a measurement or is determined mathematically based on individually measured geometrical data of the capillary and material parameters.